Proceedings of the 4th African Rift Geothermal Conference Nairobi, Kenya, 21-23 November 2012
61
Small-Scale Rural Electrification and Direct Use of Low-Temperature Geothermal
Resources at Mbaka Fault in SW Tanzania
Michael Kraml, Horst Kreuter, Graeme Robertson and Mbaka exploration team members
Baischstraße 7, 76133 Karlsruhe, Germany
Keywords: Rural electrification, Tanzania, hot springs,
low-temperature geothermal, Mbaka fault, Karonga basin
ABSTRACT The objective of the paper is to present first evidence for
suitability of Mbaka fault’s low-temperature geothermal
systems for rural electrification projects.
The Karonga basin north of Lake Nyasa in the southwestern
part of Tanzania is characterized by the prominent NW-SE
trending Livingstone border fault and the parallel major
intra-basin Mbaka fault. Several hot springs emanate along
the highly permeable Mbaka fault with temperatures of up
to 70°C, flow rates of up to 20-40 l/s and partly artesian
outflow. Three license areas are currently under geological,
hydrological and geochemical exploration by Geothermal
Power Tanzania Limited and include both Mbaka and
Livingstone faults. Fluid geothermometric results indicate
anticipated temperatures of the deep reservoir(s) of ≥160°C.
The recharge area of the Mbaka hot springs is located
mainly in the elevated region of Kiejo volcano N to NE of
the hot springs and is characterized by humid climate.
A first conclusive conceptual model is presented for the
southernmost Kilambo prospect at Mbaka fault as basis for
identification of sites for drilling of exploration wells and
planning of well paths. After successful drilling of a more
than 600m deep exploration well and a shallower injection
well, a wellhead generator will be used for early power
generation. In addition direct heat utilization will be
considered mainly for use in drying of agricultural products
by constant and reliable conditions to enhance the quality of
the products (e.g. tea leaves) or to avoid wastage of rotten
fruits (e.g. bananas). The presented rural electrification
concept will be applied in several sites at the Mbaka and
Livingstone faults if on-going exploration will show
promising results.
The paper gives an overview on previously existing data
and the new findings for geothermal development at Mbaka
and possibly also Livingstone faults.
1. INTRODUCTION It is long known that Tanzania has a significant yet
untapped geothermal potential (McNitt 1982). Recently, the
Ministry of Energy and Minerals recognized that there is
potential for enhancing the power supply security and
covering the rising power demand in the country (Mwihava
et al. 2004).
Geothermal Power Tanzania Limited (GPT) is a company
based in Dar es Salaam with shareholders from Mauritius
(Aspac Mining Limited), Germany (GeoThermal
Engineering GmbH), the Tanzanian Government’s National
Development Corporation, a local Tanzanian company -
Interstate Mining and Minerals Ltd and investors from
Australia and Singapore. GPT plans to develop geothermal
projects in Tanzania. Five prospecting licenses were
granted by the Tanzanian Ministry of Energy and Minerals
to GPT in the last quarter of 2011 in Mbeya area in
southwestern Tanzania. Together with other conventional
and renewable sources of energy, GPT plans to power the
entire Mbeya energy region. The main reasons for choosing
Mbeya area is the availability of natural resources and the
good infrastructural framework.
New exploration activity by Mbaka exploration team
started with site visits in January 2012 mainly for purposes
of identifying sites for drilling of deep temperature gradient
wells in the well-known northern part of Mbeya area. This
was followed by a more extensive field survey in May 2012
focusing on identification of “new” i.e. so far not sampled
hot springs and flow-rate measurements in the less well
studied southern part of Mbeya area. A third survey (mainly
sampling campaign) took place in July/August during the
dry season in the southern part of Mbeya area for purposes
of receiving least diluted fluids and least atmospherically
contaminated gas samples.
This area with low-temperature geothermal resources is
suitable for rural electrification.
2. RURAL ELECTRIFICATION AND DIRECT USE The project area is located within a rural area except for
Tukuyu city with ca. 50,000 inhabitants. Connecting more
village residents to electricity (rural electrification) would
mainly serve the demand for cost-effective lighting of
rooms using bulbs. The driving force for the rural
electrification process is the fact expensive kerosene for
fueling lamps becomes unnecessary (Figure 1). This would
e.g. allow pupils to more time to learn and do homework in
the evening and reduce eye and respiratory system health
problems. Additional electricity demand exists for
powering radios (currently with small non-rechargeable
dry-cell batteries) and charging mobile phones.
The major barrier to the use of electricity among the poor
village residents is the comparably high connection costs
per household. However, schemes with pre-financing of
connection costs can make those initial costs affordable.
Furthermore the project will create short- term jobs for
construction workers (road, drilling site, plant) and long-
term jobs e.g. for local guards and technical personnel
working in the power plants.
All measures together will enhance the overall economic
growth in the region.
It is expected that geothermal electricity generation will be
higher than the demand of surrounding rural households
alone. Therefore, agroindustry i.e. tea companies and small
scale industry including hotels in Tukuyu with relative
constant and comparably high demand are the second target
group for electricity and heat sales. Additionally, a diesel
powered coal mine near the border of Malawi will also be
included in the market assessment. Finally, a cross-border
33 kV transmission line would allow for export of excess
power to the nearby Malawi (interconnection between
Kyela, Tanzania to Karonga, Malawi).
Currently, the projected power demand is being estimated
and compared with the expected geothermal power
generation for solid planning of the rural electrification
project.
Figure 42: Kerosene lamps offered in shops of villages in
the project area (January 2012).
In parallel, concepts for direct heat use are under
development. Waste heat from the power plant could be
used for: (i) sustainable drying of tea leaves within an area
of major tea plantations in an effort to reduce the CO2
footprint of the tea companies, (ii) drying of cocoa seeds
under constant conditions (especially in the rainy season
when drying with the help of sunshine is not reliable) to
enhance the quality of the produced cocoa oil (Figure 2)
and (iii) drying of bananas to reduce wastage resulting from
rotting i.e. promoting banana chips production. So far only
a small amount of rotten bananas is used as ferment in
local honey breweries (Figure 3). The same fruit drying
installations could be used for e.g. mangos which cannot be
completely consumed by local villagers or sold on the
market during the time of over-production (Figure 4).
Figure 2: Cocoa seeds drying during the rainy season
(January 2012) with the help of sunshine under
non-reliable conditions (background left) and
one of several small scale cocoa oil production
sites in the foreground.
Figure 3: Honey brewery using rotten bananas as
ferment (January 2012). Left: ongoing
fermentation process; Right: final product.
Figure 4: Ripe mangos being consumed by local
villagers or sold on the market. Both measures
are not sufficient for mitigating overproduction
(January 2012).
Additionally, the temperature of the produced thermal water
would allow for effective cooling via absorption chillers
(combined heat, cooling and power plant) and this would
prevent the rapid degradation process of locally consumed
perishable foodstuffs. All measures would be beneficial for
the economic growth in the region.
3. GEOSCIENTIFIC ASSESSMENT In this chapter the geoscientific data is presented for
characterizing the low-temperature geothermal system. A
preliminary assessment (geology and geochemistry) of the
entire Mbeya area is given in Delvaux et al. (2010) and
Kraml et al. (2010).
3.1 Geology The project area is situated at the triple junction of Rukwa,
Usangu, and Karonga rift basins which form a part of the
western branch of the East African rift system (Figure 5).
The rift-rift-rift triple junction is characterized by
prominent alkaline young-Quaternary volcanism (Rungwe
volcanic complex). The Rungwe volcanic complex consists
of three major eruption centers, namely Ngozi, Rungwe
(2960m a.s.l.) and Kiejo volcanoes. The last eruption of
Kiejo volcano occurred in historic times and was dated by
reconstructing the family tree of a local villager at about
1800 A.D. (Harkin 1955).
3.1.1 Tectonic Framework The northernmost sub-basin of the Nyasa rift - the Karonga
basin - is an asymmetrical down-to-the-east half-graben,
which is characterized by the prominent NW-SE trending
Livingstone border fault and the parallel major intra-basin
Mbaka fault (both seismically active). Especially in the
latter of the two, deep-reaching structures are highly
permeable allowing for ascent of hot water from the floor of
the ca. 5km deep rift basin. Several hot springs emanate
along the Mbaka fault (Figure 5).
The springs and carbon dioxide (CO2) emanations are
located mainly at the crossing of the SW dipping and NW-
SE trending Mbaka fault with sub-vertical faults of the
youngest NNE-SSW Usangu trend and the older ENE-
WSW basement trend (Delvaux et al. 2008; Figure 6).
Figure 5: Simplified geological framework conditions in
the project area.
Usangu basin
Ngozi
Rungwe
Kiejo
50 km L. Nyasa
Malawi
Tanzania
Volcano
Hot spring
Major fault zone
Figure 6: Mapped lineaments (blue: basement trend,
red: main rift trend, orange: Usangu trend), hot
springs (pink), cold springs (blue), gas vents
(yellow), volcanic vents (green) and earthquake
epicenters (white dots). (Delvaux et al. 2010)
During Cenozoic rifting, the Rungwe area was uplifted and
denudation processes exposed the Precambrian basement at
the earth surface in Tertiary times (van der Beek et al.
1998).
The first Cenozoic rifting episode took place from Late
Miocene to Late Pliocene and is characterized by normal
faulting (NW-SE main rift trend). The subsequent rifting
period started in Middle Pleistocene with changed
kinematic regime. This Neotectonic period (which
continues until today) is characterized by a dextral strike-
slip reactivation of the NW-trending rift faults (Delvaux et
al. 1992, Mortimer et al. 2007). For a detailed recent
assessment of the complex actual stress field in the study
area see Delvaux & Barth (2010).
3.1.2 Rock Types (Basement and Volcanic/Sedimentary Infill) The Precambrian basement is exposed on the footwall of
the Mbaka and Livingstone faults. The basement consists
mainly of Ubendian gneiss, but also migmatite, quartzite
and subordinate granulite and metabasite occur.
The Ubendian gneiss is unconformably overlain by coal
bearing Karoo sediments (mainly Permian) which are
exposed at the western side of Karonga basin in the area of
the Kiwira coal mine (Songwe-Kiwira Karoo basin;
Damblon et al. 1998).
Karoo conglomerates are unconformably overlain by the
so-called “dinosaur beds” which mainly consist of red sand-
and siltstones. The thickness near Mbaka fault is not exactly
known. Further to the west a thickness of nearly 600m is
documented for those deposits (Ebinger et al. 1989, 1993a).
The stratigraphic context had been under debate for a long
time until Damblon et al. 1998 and Roberts et al. 2004
pinned down the stratigraphic relations (lower unit =
Cretaceous; upper unit = Paleogene).
The “dinosaur beds” are unconformably overlain by the
basal unit (thin muddy sandstone) of a volcano-sedimentary
sequence. This basal Late Miocene unit marks the start of
the Cenozoic rifting period.
The entire volcano-sedimentary sequence has a thickness of
>200m as documented in the western part of Karonga basin
(Ebinger et al. 1989, 1993a). However, in the central part of
Mbaka fault the exact thickness of this sequence is not
known.
Table 1: Stratigraphic column of the hanging wall of
Mbaka fault as expected at Kilambo geothermal
prospect (Ebinger et al. 1989, 1993a, Fontijn et al. 2011).
Recent top soil
Holocene Rungwe pumice 4 ka (≤1m to ≤30cm depending
on distance to eruption center)
Older “basalt” dated at 2.3 Ma at the top of the section
Ugali tuff (phonolitic ignimbrite) near the top (caps Chiwondo beds)
East of Mbaka fault: Masukulu phonolites and phonolitic tuff dated at 5.5 Ma possibly occur also W of Mbaka fault
Older “basalts” dated at 6.3 Ma
200m-thick fluvio-lacustrine sequence of cross-bedded conglomerates, sandstones, black mudstones, water-deposited ash and tuffs, and phonolitic ignimbrite
Lacustrine calcite-cemented sandstones and/or conglomerates with reworked Karoo and phonolite clasts overly the Songwe tuff
Songwe tuff dated 8.6 Ma at the base of the >200m thick section (conformably over basal unit)
basal thin muddy sandstone
Cretaceous/Paleogene “dinosaur beds” (mainly red sand-
and siltstones) <600m
Permian Karoo Supergroup (conglomerates, coal bearing
fine grained sediments)
Precambrian basement (mainly Ubendian gneiss,
migmatites, quartzites)
The expected stratigraphic succession at the hanging wall of
Mbaka fault is presented in Table 1 including the detailed
stratigraphy of the Late Miocene to Late Pliocene sequence.
On the footwall of Mbaka fault, the young volcanic
eruption products of Rungwe volcanic complex are directly
overlying the Precambrian basement. The major rock types
of the mafic alkali-basaltic lavas are nephelinites (at Kiejo),
basanites (at Tukuyu), alkali basalts (at Rungwe) and
picrites (at Kiejo). Differentiated rocks include phonolites
and trachytes also in the form of pumice, the latter having
formed during highly explosive (up to plinian) eruptions.
The volcanic activity can be divided into three phases based
on field relations (Harkin 1960) and on age determinations
(Ebinger et al. 1989, 1993a; Ivanov et al. 1999): (i) Late
Miocene - 9.2-5.4 Ma, (ii) Late Pliocene to Early
Pleistocene - 3-1.6 Ma, (iii) Mid-Pleistocene to Recent -
since 0.6 Ma. The first two phases correspond to the Older
Extrusives and the third phase to the Younger Extrusives, as
defined by Harkin (1960. See Fontijn et al. 2012 for a
detailed recent volcanological review of the study area).
L.
Nyasa
At the high-priority site Kilambo, alkali-basaltic lavas (2.3
Ma) and trachytic to phonolitic tephra layers form the
uppermost part of the stratigraphic succession except for a
thin cover with 4 ka Rungwe pumice (Figure 7) and the top
soil (Table 1). Directly at the adjacent escarpment of Mbaka
fault a young alkali-basaltic lava flow of Kiejo volcano is
exposed in a quarry for mining of road construction
material. The young lava flow has taken the same valley as
the straight river both following in a newly postulated fault
perpendicular to Mbaka fault (Usangu trend).
Figure 7: Isopach map [values in cm] for the 4 ka
trachytic Rungwe pumice (from Fontijn et al.
2011; modified). The red triangle marks Rungwe
crater and the three red stars mark the major
geothermal systems at Mbaka fault.
The coincidence of abundant maars aligned parallel to the
southern part of Mbaka fault and along perpendicular
magma feeding directions of Kiejo (see Figure 20 below)
suggest that the pathways of descending surface waters and
ascending magma injections are both fault-controlled.
3.2 Geochemistry In this section a short overview is given on the chemical
and isotopic composition of fluids, gases and travertines.
For a more extended discussion (except for newly sampled
warm spring at Livingstone fault) see Kraml et al. (2010).
3.2.1 Fluid and Gas Chemistry Major element geochemistry (Figure 8) indicates that the
sampled Na-HCO3(-Cl) fluids of the Mbaka hot springs as
well as warm spring at Livingstone fault are “peripheral
waters” with a high amount of admixed (near-) surface
water. Therefore, also in the Na-K-Mg diagram (Figure 9),
all fluids are plotting in the immature or partly equilibrated
field. That excludes a geothermometric assessment for
Livingstone sample. However, for Mbaka and western
Karonga fluids, reservoir temperatures of >160°C can be
deduced based on mixing of an equilibrated hydrothermal
fluid with (near-) surface waters. These findings are in
agreement with earlier studies (e.g. Hochstein et al. 2000,
Branchu et al. 2005, Delalande et al. 2011).
First geochemical modeling (Mnjokava 2007 and Kraml &
Hehn unpublished) of Kilambo hot spring water implies a
mixture of hot ascending reservoir fluid with cold near-
surface water in mixing ratios of about 1:5. Total dissolved
solids of the modeled reservoir fluid are expected to be
around 20 g/l and the fluid is oversaturated in some silicate
minerals and silica (see also modeling results of Delalande
et al. 2011 for Ilwalilo hot springs [named Mbaka in the
present paper and in Kraml et al. 2008, 2010] located at
Mbaka fault NW of Kilambo circulation system).
Figure 8: Cl-SO4-Alkalinity diagram indicating a high
proportion of (near-) surface water of hot springs
and especially of newly sampled warm spring at
Livingstone fault.
Figure 9: Na-K-Mg diagram showing mixing relations
of a deep equilibrated fluid with (near-) surface
waters.
Trace element geochemistry of the fluids including Rare
Earth Elements (REE) was first applied in the study area by
Kraml et al. (2008). The REE pattern (Figure 10) shows a
characteristic positive Eu anomaly only in the case of hot
spring waters indicating plagioclase alteration during the
interaction of hot fluid with the reservoir rock (gneiss).
The river water is characterized by LREE enrichment, flat
HREE patterns and the ubiquitous negative Eu anomaly as
well as higher REE concentrations which is typical of upper
continental crust and clastic sediments (McLennan 1989).
The same pattern and very high concentrations are
characteristic of highly differentiated alkaline pumice
deposits from Rungwe volcanic complex as analyzed in a
<1000 year old distal ash layer intercalated in sediments of
northern Lake Nyasa (Williams et al. 1993).
Alkali basaltic rocks occurring in the area (i.e. alkali
basalts, nepelinites, basanites) show steep patterns from La
through Tb and slightly decreasing HREE pattern (Furman
1995).
Figure 10: REE pattern of Karonga hot springs
compared to a river and a cold spring in the
study area (normalized with values of Sun &
McDonough 1989) (see text).
The cold spring - Mulagara - (Figure 10), which emanates
from the southern flank of Rungwe volcano, shows a
similar REE concentration range to the hot springs but no
Eu anomaly and a rising HREE pattern. Most cold springs
in highly elevated areas at Rungwe and Kiejo volcano
represent perched groundwater aquifers.
Fluid evolution (see extended discussion in Kraml et al.
2010) is characterized by fault-controlled descending
meteoric surface waters (gravity driven), enriched by
ascending volcanic CO2. The high CO2 flux in the recharge
area is documented by commercial shallow CO2 wells
(0.5bar) drilled for the production of sparkling soft-drinks.
Major species gas analyses of the free gas phase indicate
that the gas composition in the hot springs can be explained
by simple two component mixtures i.e. volcanic CO2 and
atmospheric gas (Figure 11).
The CO2-rich water causes silicate hydrolysis of the
crystalline rocks i.e. alkali-basaltic cover and gneisses of
metamorphic basement (non-equilibrium conditions with
excess SiO2 and HCO3-). The fluid tends to equilibrate in
the deep reservoir within Precambrian gneisses (as
supported by a crustal He and Sr component; see below).
Ascending hot fluid (buoyancy driven) partly re-
equilibrates at lower temperatures (110 to 120°C) on the
way to the surface (as indicated by fast equilibrating silica
and K/Mg geothermometers), degasses at relative shallow
level (calcium carbonate precipitation) and mixes with
shallow groundwater (non-equilibrium). The CO2 in the
shallow groundwater is also overwhelmingly of volcanic
and not atmospheric origin excluding CO2 used as tracer for
mixing processes (see section 3.2.2).
Figure 11: Mixing of volcanic and atmospheric gas. The
fraction of atmospheric gas depends on the
admixed fraction of air-saturated (near-) surface
water (Kraml et al. 2010). 3.2.2 Gas Isotopic Composition Gas samples of bubbling hot and cold springs were also
analyzed for their carbon isotopic composition of CO2. The
measured values are characteristic for volcanic CO2 (Kraml
et al. 2010). Also the 14C activity values of TDIC and CO2
gas indicate a “dead” carbon input, without any significant
contamination by atmospheric carbon (see Delalande et al.
2011 for extended discussion).
Additionally, helium isotopic compositions of the free gas
phase were analyzed. Water sample was taken which may
have been atmospherically contaminated during sampling
procedure, only in case of Kilambo (as noted in the field
book and in Kraml et al. 2008). The results are presented in
Figure 12.
Figure 12: Normalized Helium isotopic composition vs.
Ne/He ratio (data were taken from Pik et al. 2006
and Kraml et al. 2008).
The helium isotopic composition of most samples can be
explained by a mixture of a mantle and a crustal
components pointing to a long residence time in the
Precambrian basement. Only the water sample from
Kilambo hot spring and a nearby sample of gas bubbles
taken from a river near Makwehe hot spring, showed a
mixing relation between the Kiejo mantle component and
the atmosphere. However, comparable data from Kilambo
springs analyzed by Pik et al. 2006 did not show
atmospheric mixing trend. Therefore, new helium samples
will be taken for purposes of clarifying this important issue.
3.2.3 Travertine Isotopic Composition Travertine of Kilambo hot spring consisting of calcite (as
determined by XRD analyses) was analyzed for its stable
isotope composition of Strontium, Oxygen and Carbon.
Using the 18 13C value of the
CO2 in the gas phase and the fractionation factors of O’Neil 18O
CaCO3-H213C CaCO3-CO2 respectively for
calculating the paleo-fluid temperatures, the values are
close to the present hot spring temperature (54°C and 53°C
compared to the measured 56°C).
A systematic study of fossil occurrences of Kilambo
travertine (U/Th dating and stable isotopes) would be
necessary to reconstruct the geothermal activity in space
and time.
The strontium isotopic composition of Kilambo travertine
indicates a more intensive fluid/rock interaction with the
Precambrian gneiss than the Songwe fluids (and related
travertines; Figure 13). The latter belong to the separate
geothermal system of Ngozi volcano further to the north. In
accordance with high crustal helium components (which is
yet to be confirmed for Kilambo) and the pronounced
positive Eu anomaly in the REE pattern, independent
constraints could be put on the type of the reservoir rock.
Figure 13: Sr-isotopic composition of basement rocks
(Ubendian gneiss), travertines and Songwe hot
spring water (for explanation see text).
3.3 Climate and Hydrology In this section, a short summary of climatic and hydrologic
conditions of the study area is described below.
3.3.1 Climate The ratio of potential evapotranspiration to precipitation
(index of dryness) in the target region of Mbaka and
Livingstone fault including the recharge areas is 0.6
indicating humid conditions (catchment 1.11, Figure 14).
The humid climate assures a comparably high recharge
maintaining the water pressure in the deep reservoir.
The overall hydraulic gradient in the drainage basin is
directed to the SSE (Gibert et al. 2002).
Figure 14: Mean annual potential evaporation,
precipitation and runoff in study area situated
within drainage basin 1.11 (CCKK 1982). 3.3.2 Hydrology and Isotope Hydrology Kilambo hot springs are characterized by a combined flow
rate of 20-40 l/s (Hochstein et al. 2000) and partly artesian
outflow (Figure 15 a,b). The temperature of the hot springs
and their artesian pressure (if any) vary with time and
season. This observation supports the model with final
admixture of a local groundwater component.
Figure 15a: Gas-rich hot spring in Kilambo area with
slightly artesian outflow (water temperature of
61.1°C on 14th
of May 2012). The flow rate was
measured at slightly above 1 l/s with a 28°V-
notch weir.
Figure 15b: Spouting spring in Kilambo area with
slightly artesian outflow (water temperature of
55.3°C on 14th
of May 2012). Flow rate estimated
at significantly above 1 l/s.
The working area is surrounded by highlands of significant
elevation (Figure 16).
Figure 16: Highlands around working area situated SE
of Tukuyu.
The highlands are characterized by a depleted oxygen and
hydrogen isotopic composition (altitude effect) and lakes
like Lake Nyasa in the lowland show an additionally
enriched signature due to their free water surface (=>
evaporation) (Figure 17).
Figure 17: Areal distribution of
working area during dry and rainy season (data
from Delalande et al. 2005 to 2011, Branchu et al.
2005 and Kraml et al. 2008). Artifacts from the
interpolation algorithm could be avoided at Lake
Nyasa where data points with the same value
were added (location not shown)
Since Kilambo fluids are characterized by depleted isotopic
compositions the recharge area must include the highland of
Kiejo volcano in the north. Kiejo water samples taken from
two rivers at 1625m and 491m a.s.l. and from one cold
spring located at ca. 920m a.s.l. are slightly more depleted
than Kilambo hot spring fluids (Figure 18) which emanate
in ca. 600m a.s.l.. Most depleted fluid samples are from two
rivers originating near Rungwe summit and one spring
located near the summit of Rungwe volcano.
Shanzira cold spring (2500m a.s.l.) near the summit of
Rungwe shows a slight oxygen isotope shift to the left of
the Meteoric Water Lines (both local and global) which can
be explained by isotope exchange with volcanic CO2. This
exchange reaction is not restricted to subsurface waters and
also appears at rivers (e.g. Shiwaga: >2‰ oxygen isotope
shift; Figure 18 and Delalande et al. 2011) where the river
crosses a fault characterized by a massive CO2-degassing
(see also section 3.5).
18O diagram for fluids sampled in
the study area (dry season). Data are taken from
the extensive work of Delalande 2005 to 2011 as
well as from Branchu et al. 2005 and Kraml et al.
2008 (for explanation see text).
The high amount of CO2 involved in the evolution of the
hot spring fluids also leads to a significant shift in isotopic
composition to the left of the Meteoric Water Lines (H2O-
CO2 exchange effect up to >7‰!; see Delalande et al. 2011)
masking the equilibrium achieved in the deep reservoir
during oxygen isotope exchange with the gneiss (where
water/rock interaction at high temperature took place
causing a less pronounced shift to the right). The higher
shift to the left of the MWLs in case of more saline hotter
fluids could be attributable to the flushing with volcanic
CO2 once the fluid meets the highly permeable Mbaka fault
in depth. The exchange reaction between water and CO2 is
very fast even in the vapor phase (Delalande et al. 2011).
Waters ascending fast in highest permeable “CO2-lifted”
zones show only negligible admixture of surface (river)
water containing atmospheric CO2. The river water
infiltrates only in little quantities at Mbaka fault from the
earth surface (with H- and O-isotopic mean composition of
rain in Rungwe area in 900m elevation) and/or groundwater
with evaporation signature (like Katunduru-Masoko cold
spring trend) meets the uprising fluid. The lower the
amount of admixed river water and the lower the
evaporation signature (of only locally occurring ground-
water at Masoko maar-lake) the more is the left shifted
composition preserved compared to stronger diluted fluids.
3.4 Conceptual Models
Hochstein (2000) presented the first conceptual model for
the Kilambo hot spring, i.e. outflow of a concealed
(possibly high-temperature) reservoir further upstream (i.e.
in 18km distance). As inferred from the location of the
reservoir, he suggested the only known place with silica
sinter (described by Harkin 1960) in the entire Mbaka
region.
Branchu et al. (2005) interpreted the Kilambo and two other
hot springs (Mampulo & Kasimulo) at the western rim of
the Karonga basin as basinal brines rising from the deepest
part of Karonga basin from about 5km depth and diluted
with meteoric water. Those authors suggested a magmatic
heat source as the driving force and explained the elevated
heat flow measured at the bottom of northern Lake Nyasa
with values at the western part of up to 80-90mW/m2. The
magmatic heat source is responsible for a fault related
localized hydrothermal discharge at the lake floor and its
extension on land is represented by Kasimulo hot springs.
Along the lake axis the heat flow with values of 50-
60mW/m2 is still slightly higher than the mean value for the
African continent (49.8 mW/m2). There, at least sub-
lacustrine gas emanations in 4m water depth were observed
near the mouth of Kiwira river; Kyela lineament in Figure
6).
In this paper we interpret the three separated major hot
spring areas at Mbaka fault as follows (Figure 19): In the
higher elevated areas of Kiejo and Rungwe volcano
meteoric water infiltrates and the fluid is heated up on the
way down taking up volcanic CO2 (high flux in the Rungwe
and Kiejo area). The hot fluids are mainly guided along
active sub-vertical faults of the Usangu trend and along
faults of the Basement trend to Mbaka fault (rift basin
trend) in the SW of the recharge area. The hot fluids rise at
the deep entry point at Mbaka fault leading to surface
manifestations in the form of hot springs along the fault
trace (dilution by descending river water at Mbaka fault and
shallow ground water).
The fossil silica sinter on the basement gneiss at the
northernmost geothermal system could be explained by
additional magmatic heat released e.g. during the most
prominent Holocene eruption of Rungwe volcano (4 ka)
due to the recent replenishment of the magma chamber
leading to excess conductive heat. For the southernmost
high priority Kilambo prospect, a conclusive conceptual
model has to include the magmatic heat from Kiejo
volcanic system in the evolution of the fluids (Figure 20).
Figure 19: Schematic sketch of fault-hosted Kilambo
geothermal circulation system (not to scale). The
Kilambo hot springs are aligned along Mbaka
fault and the main recharge area with
infiltration of surface water (indicated by blue to
orange arrows) is located just outside of the
block model to the right. The additional volcanic
heat source (recent dike intrusions of Kiejo
volcano) is not shown for clarity.
Figure 20: Dotted grey lines suggest magma feeding
directions of Kiejo volcano based on vent
alignments, and vent elongation directions
(Fontijn et al. 2010, modified)
3.5 Preliminary Hazard Assessment Natural hazards are mainly due to inter-related seismicity,
volcanic activity, CO2 degassing and landslides in case of
steep volcanic topography with thick unconsolidated
pumice deposits, being typical for Rungwe volcano.
Since three zones of seismicity meet within the working
area, the activity rate of major earthquakes (= annual
number above the lower bound magnitude of 4.5) gives the
best case values of 1 earthquake every 2.4 years with a
magnitude of up to 7.1, most likely 1 earthquake every 1.4
years with a magnitude of up to 7.3 and in the worst case 3
earthquakes per year with a magnitude of up to 7.8 (Midzi
et al. 1999; see also Langston et al. 1998).
Landslides could easily be triggered by seismicicity or
volcanic activity in Rungwe area but also by liquefaction of
pumice deposits after heavy rain falls (Bucher 1980).
A continuous volcanic risk is based on CO2 emanations
especially along active faults, causing human victims like at
site Shiwaga river located NW of Rungwe volcano.
In addition to explosive eruptions, debris avalanche
deposits of a major sector collapse of Rungwe volcano
masks the trace of the northern prolongation of Mbaka fault
at the earth’s surface (Fontijn et al. 2010). Alkali-basaltic
lava flows like the 8km long only 200 years old Sarabwe
flow from Kiejo volcano (Harkin 1955) have to be taken
into account for planning the geothermal installations.
Dike-like magma injections of Kiejo volcano, which is
characterized by dominantly alkali-basaltic effusive
activity, are expected at Mbaka fault near the Kilambo
prospect (Figure 20).
Only Iramba maar-lake nearest to Mbaka fault, less than
2.5km SE of Kilambo hot spring and located at the end of
the major magma feeding system of Kiejo, shows a
contribution of deep hot fluid (Delalande et al. 2008b). The
calculated hydrothermal input corresponds to 3-4 l/s of
Kilambo spring water. Other maar lakes aligned parallel to
Mbaka fault further in the foreland (Figure 20) may reflect
former injections of Kiejo’s magma feeding system. In that
respect the not investigated Lake Ikapu 2km west of
Kilambo hot spring will be included in the subsequent
investigations to fully understand the entire Kilambo
circulation system.
In the future one could learn from a continuous monitoring
of hot spring temperature, spouting height, electrical
conductivity in combination with chlorine measurements
how pronounced the seasonal effect is for using the
remaining signal for hazard assessment.
3.6 Preliminary Drilling Concept and Plan The preliminary drilling concept aims to tap the hot rising
fluid below the mixing zone with local groundwater and
above the entry point in depth. Up to three additional
production wells are possible at Kilambo site to enhance the
yield in case of a low flow rate. After successful drilling of
the production well(s), a reinjection well will be drilled
downstream (= in SE direction) to a depth shallower than
the production well i.e. within the upper part of the mixing
zone of groundwater with hot fluid.
Based on the expected stratigraphy in the hanging wall of
Mbaka fault the preliminary drilling plan for the first more
than 600m deep wildcat slim hole using an Atlas Copco
CS14 rig is as follows:
A conductor pipe (e.g. in the first 5 meters) with well head
for lateral outflow of drilling mud and thermal water
(diameter PQ) will be set into the ground to provide the
initial stable structural foundation for the borehole. For the
first few hundred meters (i.e. the thickness of basaltic lavas
and tuffs) drilling with HQ diameter using standard wire-
line drilling equipment is planned. The depth drilled with
HQ diameter will be 1.5m into the red sandstone for stable
placement of the NW casing. The next several hundred
meters (= mainly sandstones and intercalated mudstones)
down to the target (= gneiss) drilling with NQ standard
wire-line equipment is envisaged. After logging, testing and
sampling the drilling rig will be removed and the
exploration well will be available for later long-term
production tests and for monitoring while drilling the full-
size production well some 10 meters aside with a bigger rig.
At the very end of the project once the observation well is
no longer neede the well will be cemented (a few hundred
meters) down to the casing shoe of the NW casing, to leave
the it in an environmentally safe condition.
4. CONCLUSIONS
The Mbaka fault bears a few low-temperature fault-hosted
deep-circulating geothermal systems, fed by perpendicular
faults on the foot wall with deep fluid which may
additionally be heated by Rungwe magma chamber in the
northern part and (less sustainably) by recent Kiejo dike
intrusions in the southern area.
Infiltrating surface waters diluting the ascending hot fluids
are provided mainly from rivers which are erosionally cut
into the hanging wall along the perpendicular faults as well
as directly at the Mbaka fault itself.
The preliminary drilling concept aims at tapping the hot
rising fluid below the mixing zone with local groundwater
and above the entry point in depth. The reinjection well will
be located downstream (= in SE direction) and in shallower
depth than the production well.
After successful drilling, a wellhead generator will be
installed for rural electrification and direct use of the waste
heat (mainly drying of tea leaves and banana chips
production).
A significant benefit for the rapidly growing Mbeya area is
expected from serving the energy needs of the region – it
will result in an enhanced economic growth in the entire
region including the rural areas.
5. ACKNOWLEDGEMENT The extensive support during field work from the Mbaka
exploration team, namely: James Sullivan (Exploration
Manager), Jim Rush (Drilling Manager), Abbas Nyangi
(Drilling Supervisor) and drivers Nelson, Nuru and Elias is
greatly acknowledged. Geoscientists René Grobe and Vera
Hehn assisted in the implementation of GIS projects and in
geochemical modeling, respectively. Additional thanks go
to the drilling engineer, Ingo Schwind for intensive
discussions on drilling issues and tectonic expert Damien
Delvaux for valuable discussions on structural specialties of
the field area. Finally, James Wambugu is thanked for his
constructive review which enhanced the overall quality of
the manuscript.
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